Two stage chemical digestion within a single vessel with wash between stages



Feb. 4, 1969 o. A. LAAKso 3,425,898` TWO STAGE CHEMICAL DIGESTION WITHIN A SINGLE VESSEL WITH WASH BETWEEN STAGES Filed Dec. 14, 1964 Shet of 2 Feb. 4', 1969 o A. LAAKso 3,425,898

TWO STAGE CHEMICAL D'IGESTION WITHIN A SINGLE VESSEL Filed Dec. 14, 1964 4 ,piqi United States Patent O 13,869/ 63 U.S. Cl. 162--19 Int. Cl. D21c 7/00 7 Claims ABSTRACT OF THE DISCLOSURE Apparatus and a process are disclosed for performing a two-stage chemical treatment on cellulosic fibrous material, particularly digestion and/or bleaching thereof, comprising a longitudinal pressure-resistant vessel of a CII substantially uniform cross-section, which is provided with means for continuous feed of fibrous material into one end and for discharge thereof out of the other end,

and further with means for supplying and withdrawing treating liquids of two different kinds at places spaced along the longitudinal direction of the vessel. This is shown achieved by supplying wash liquid at a place located between the two treatment stages and in an amount sufficient to cause a wash liquid flow counter-current to the direction of feed of the fibrous material to thereby displace the treatment liquid of the first stage and wash out the first stage treatment liquid before the fibrous material reaches the zone where it will 1be contacted by the treatment liquid of the second stage.

The present invention relates to a method of performing a two stage chemical treatment on cellulosic fibrous material during passage thereof through la continuous digester. The invention also relates to improvements in continuous digestion apparatus for performing a two stage chemical treatment on cellulosic fibrous material passing therethrough.

According to the invention, a two stage chemical treatment is one in which each stage is independent of the other to the extent that different chemical treatments, e.g., digestion and bleaching, may be performed in each stage without interference from the other, although the invention is equally applicable to the'use of two chemical treatments that are mutually miscible, e.g., two-stage digestion. It is a feature of the invention that the two chemicals employed may be of widely divergent chemical and physical properties, and that treatments therewith may be carried out continuously and simultaneously in one and the same continuous digestion vessel.

Two stage continuous digesters are known wherein a cellulosic fibrous material feed may be subjected to first digestion treatment and a second digestion treatment. However, such devices present a problem in that it is not possible to simultaneously treat cellulosic fibrous material with two different chemical treatments in one and the same pressure-resistant digestion vessel, while the cellulosic fibrous material is fed continuously therethrough. More specifically, it is not possible in presently known two-stage digesters to make the two treatment stages independent of each other, so that it would be possible to treat the cellulosic material with two kinds of liquids that are so different that mixing of them would involve difficulties and therefore should be avoided.

Longitudinal pressure-resistant digestion vessels are known in the art which have a substantially uniform cross-section and are provided with means for continuous feed of fibrous material into one end and for ydischarge thereof out of the other end. However, such vessels have ICC hithereto not been provided with further means for supplying and withdrawing treating liquids of two different kinds at spaced points along the length of the vessel such that until now it has not been possible to perform a two-stage chemical treatment, particularly digestion and/or bleaching of cellulosic fibrous material that is continuously passing through the vessel.

: `It is therefore the object of the present invention to provide to a method of performing a two-stage chemical treatment on cellulosic fibrous material while it is fed continuously through a pressure-resistant digestion vessel wherein the two treatment stages are independent of each other, so that it is possible to separately and simultaneously treat the cellulosic material with two different liquids that may be so different that mixing of them would involve difficulties, e.g., inter-reactions, that are desired to be prevented.

It is another object of this invention to provide apparatus for performing a two-stage chemical treatment on cellulosic fibrous material, particularly digestion and/ or bleaching thereof, comprising a longitudinal pressure-resistant vessel of a substantially uniform crosssection, which is provided with means for continuous feed of fibrous material into one end and for -discharge thereof out of the other end, and further with means for supplying and withdrawing treating liquids of two difierent kinds at places spaced along the longitudinal direction of the vessel.

Other and further objects of this invention, together with an Iappreciation for the advantages thereof will become increasingly apparent as this description proceeds.

Broadly stated, the foregoing objects are achieved according to the invention by supplying Wash liquid at a place located between the two treatment stages and in an amount sufiicient to cause a wash liquid fiow countercurrent to the direction of feed of the fibrous material to thereby displace the treatment liquid of the first stage and wash out the first stage treatment liquid before the fibrous material reaches the zone where it will be contacted by the treatment liquid of the second stage.

Further, the objects of the invention are achieved by providing a two-stage digestion vessel with means for supply of wash liquid and for spreading the same over the entire cross-section of the vessel. According to the invention, such means are disposed in a zone of the vessel located between sieve girdles which are mutually spaced apart along the longitudinal direction of the vessel and each being adapted for the extraction from the fibrous material of a different kind of spent treatment liquid.

The invention will be more closely described herein below with reference to the accompanynig drawings, in which FIGURES 1 and 2 diagrammatically show two embodiments of the invention that are each adapted for performing the above-mentioned two-stage treatment of cellulosic fibrous material.

In FIGURE 1, the reference numeral 11 designates an upright cylindrical pressure-resistant vessel of a substantially uniform cross-sectional area `and a length of the order -of l0 times its diameter. At its upper end, the vessel is provided with charging means or feeding devcie 13, the construction of which is well known in the art such as from Patent No. 2,459,180, and accordingly need not be described here. By means of the device 13, a comminuted cellulosic fibrous material, such as wood chips, is charged into the vessel either continuously or as small successive batches. The fibrous material moves vertically downwards through the vessel, taking the form of a rigid Column and is discharged at the lower end of the vessel into a conduit 15 by the aid of discharging means which are also well known and therefore not shown yor described herein. The direction of feed of the fibrous material is indicated by double arrows 17. During its passage through the vessel the column of fibrous material is subjected to various treatments which take place in separate zones denoted by the reference letters a, b, c, d and e.

In the first comparatively short zone a, the chips are impregnated with digesting liquor fed into the vessel together with the chips, as indicated by the arrow 19. This liquor which is provided for the first stage of delignifying digestion of the fibrous material, may preferably consist -of sulphite liquor, e.g., sodium or calcium sulphite liquor. As the arrows 21 indicate, the first digesting liquor in this zone a moves concurrently with the fiber material. When the mixture of chips and first digesting liquor has reached a point opposite to the first sieve girdle 23 inserted into the shell of the vessel, heating of the fiber and liquor mixture takes place by extracting the digesting liquor through the first sieve girdle 23. The extracted liquor is then circulated by means of pump 25 in the first exterior conduit shown. This conduit is provided with a heating devi-ce 2.7 which is connected to an interior tube 29 extending along the axis of the digestion vessel 11 and having an orifice at or slightly above the level of the sieve girdle 23. The withdrawn liquor, which has been heatedin the heating device 27 by first stage steam supplied thereto through conduit 31, is spread from the orifice of the tube 29 radially outwards and causes the column of fibrous material to be brought to the proper digesting temperature. As an alternative embodiment, the heating may take place by injecting steam directly into the digestion vessel 11 or into the circulation conduit.

The first stage of digestion atkes place in the zone b, below the heating and impregnating zone a, so that when the fibrous material reaches the level of the second sieve girdle 33 inserted into the shell of the vessel 11, the first digesting liquor fed concurrently with the chips is almost entirely spent or depleted and the chemical treatment of the chips by the same is completed. The spent liquor is drawn off throught the sieve girdle 33 and a part thereof is pumped into the above-described first circulation loop by means of a pump 35 and is thus returned to the vessel through the orifice of the tube 29, whereas another part 4of the spent liquor is discharged through a conduit 37, e.g. leading to a plant for recovery of the heat and chemical values of the liquor. As the arrows 39 indicate, the liquor in zone b also moves concurrently with the fibrous material.

Immediately adjacent to the sieve girdle 33 or at a comparatively small distance below sieve girdle 33 the shell of the vessel 11 is provided with a third sieve girdle 40. It will be recognized that the two last-mentioned sieve girdles 33 and 40 are situated at a distance from the charging end of the vessel approximating from one fourth to one half of the total length of the vessel and preferably about one third thereof.

Connected to the sieve girdle 40 is a second exterior conduit 42 which in its turn is connected to an interior conduit 44 extending along the axis of the vessel 11 and having its orifice situated at the level of the sieve girdle 40 or at a somewhat higher level. A pump 46 inserted in the conduit 42 maintains a circulating flow in the second circulation loop thus formed. To this last-mentioned circulation loop there is also connected a conduit 43 through which a first wash liquid is supplied. The first wash liquid may consist of clean water or of such white water as is obtained as filtrate from a wash filter upon which the pulp is washed after its discharge out of the vessel 11. From the orifice of the conduit 44, the first wash water is spread radially outwards towards the sieve girdle 40 and thus is distributed over the entire cross-section of the vessel. Such a great amount of wash water is supplied through the conduit 48, that from the level of the sieve girdle 40, an upwardly directed iiow of liquid, i.e. a flow counter-current to the direction of movement `of the brous material is formed which flow passes through the fibrous material and towards the sieve girdle 33. Therefore, in its movement through the zone c, the downwardly flowing fibrous material encounters a flow of wash liquid which washes out with it any digesting liquor still remaining in the fibrous material after the latter has moved past the sieve girdle 33. Thus, residues of the digesting liquor are returned upwardly and are likewise discharged through said last-mentioned sieve girdle 33. Therefore, when the fibrous material reaches the zone d, it has been washed or relieved of the above-mentioned spent liquor so that the fibrous material subsequently can be treated with another type of treating liquid. This is particularly important where the treating liquid would not react properly in the manner intended if spent liquor remaining from the digestion in zone b were allowed to mix therewith.

The wall of the vessel is provided with a fourth sieve girdle 41 whi-ch is inserted closely underneath the sieve girdle 40. Sieve girdle 41 serves to relieve the fibrous material of such spent digesting liquor as has been used for a second stage of delignifying digestion in the zone d. By reason of the first digesting liquor and wash liquid being drawn off, and aided by the second stage wash liquid described hereinafter, the second stage delignifying and digesting liquor may be introduced with a force suiiicient to move it upwardly and countercurrently to the direction of movement of the fibrous material, as indicated by the arrows 43. At the level of the sieve girdles 41 and 40 the second stage digesting liquor encounters an oppositely directe-d fiow of liquid, such that the second stage digesting liquor must be drawn off through the sieve girdle 41. lt is then evident that a stagnant border layer exists between the sieve girdles 33 and 41 at a region formed by the converging of the oppositely directed liquid iiow patterns. Dependent upon the location of this border layer, either the first wash water from the conduit 48 may penetrate downwardly and bring with it residues of the digesting liquor of the second stage towards the sieve girdle 41, or the residues of the digesting liquor of the second stage may be carried with the liquid circulating in the second circulation loop 40-46-42-44 and discharged through the sieve girdle 33.

The second digesting zone d extends from the sieve girdle 41 to a fifth sieve girdle 45 which is situated at a distance from the outlet end of the vessel 11 amounting approximately to one third of the vessel length. Sieve girdle 45 is connected into a third circulation loop comprising a heat exchanger 47 and a tube 49 located centrally of the vessel and having at its end an orifice situated at the level of the sieve girdle 45 or somewhat higher. The pump 51 brings the liquid to circulate in thsi third circulation loop, and chemicals coming from the conduit 53 and required for the second digesting stage, e.g. sulphate liquor or SO2, are charged into this third circulation loop. Second stage steam from the conduit 55 is supplied to the heating device 47 in order to heat the circulating liquid to the temperature required for the second digesting stage.

Washing of the digested fibrous material is performed in the lowermost zone e of the digester 11 by means of second stage wash liquid which usually consists either of water or of filtrate coming from a filter washing plant and containing residues of digesting liquor. The second stage wash liquid is supplied through spray nozzles 57 distributed around the pulp outlet 59 of the vessel 11. A portion of the second stage wash liquid, which also performs a cooling function, accompanies the pulp out through the outlet 59, and another portion of the second stage wash liquid is driven upwardly through the pulp or fibrous material column countercurrent thereto, as indicated by the arrows 61. The second stage wash liquid displaces the second stage digesting liquor spread from the orifice of the tube 49 and forces the second stage digesting liquor to move upwardly and countercurrent (arrows 43) to the movement of fibrous material (arrows 17). At the level of the sieves 40, 41 the second stage digesting liquor ow encounters the first stage wash water supplied through the orifice of the conduit 44, whereby the second stage digesting liquor, possibly in admixture with first stage wash liquid, is forced to depart through the sieve girdle 41. From there the withdrawn Wash liquid is led to a tank 63, eg., a cyclone, in which a pressure considerably reduced in comparison to the digester pressure is maintained and from which the released steam or gas is carried ofi through the conduit 65 to a device (not shown) for recovery of heat therein, whereas the spent liquor is led through the conduit 67 to a plant for recovery of the chemicals therein.

When the digestion of the second stage is performed with SO2 supplied through the conduit 53 in gaseous form or as SO2-water, such gas departs through the conduit 65 and can 'be recovered for repeated use in the digester.

The apparatus or digestion plant shown in FIGURE 2 corresponds in large part to the plant shown in FIGURE 1, and the same reference numerals have accordingly been employed to denote the same or similar corresponding parts. The various circulation loops are provided with different piping arrangements for clarity, although their operation remains unchanged. Thus, as with the apparatus of the FIGURE l embodiment, the zones a, b and c serve for impregnation, delignifying digestion and counter-current wash, respectively. In this embodiment however, a further sieve girdle 34 has been added which is situated closely below the sieve girdle 33t. Connected to sieve girdle 34 is an intermediate circulation loop consisting of the exterior conduit 28 with pump 30 and an interior conduit 32 which extends along the axis of the vessel 11 and has an end orifice situated at the level of the sieve girdle 33 or somewhat higher. Due to the circulation in the intermediate circulation loop, a heavy radial flow is maintained in the section lying between the sieve girdles 33 and 34, whereby the removal of the rst digesting liquor through the sieve girdle 33 becomes still more complete. The spent digesting liquor departs through the conduit 37 to a tank, e.g. `a cyclone, in which a comparatively low pressure is maintained and from which the released steam or gas is led off through the conduit 20, whereas the spent digesting liquor departs through the conduit 22 to a plant for recovery of the chemical values. Inserted in the conduit 37 yare heat exchangers 24 and 26 by means of which the -heat of the spent liquor is utilized for heating of the first stage wash water supplied through the conduit 48 and used in the washing zone c, and also for heating of the second stage wash water supplied to the lowermost end of the vessel 11.

In t-his embodiment, the fibrous material is completely delignified during treatment in the zone b, and the operation performed in zone d consists of a bleaching operation brought about by a solvent which is volatile at atmospheric pressure Ibut is in liquid form at the pressure prevailing in the |vessel 11. This solvent is supplied through the conduit 53. The heat exchanger -47 which in this case is inserted in the conduit connecting the sieve girdle 41 to the cyclone 63, transfers heat to the solvent circulating in the third circulation loop 4S-51-47-49. From the end of tube 49, the solvent moves upwardly counter-current to the direction of the fibrous material or pulp and towards the sieve girdle 41, while dissolving coloring substances in the pulp. The main part of the solvent departs through the sieve girdle 41 to the tank 63 in which a lower pressure is maintained, so that the solvent evaporates. Thus the solvent is separated from the products dissolved thereby, and the solvent is then returned for repeated use in the vessel 11. v

The apparatus illustrated in FIGURES 1 and 2 is also susceptible to another modification which consists in that the second stage cool wash water supplied through the nozzles 57 to the lowermost end of the vessel 11, is discharged through a further lowermost sieve girdle 50 and is carried by a pump 52 through an outer conduit 54 in which the heat exchanger 26 is inserted, back to an inner conduit 56 which lets out the heated water through its open end located approximately -at the level of the sieve girdle 50, thereby raising the temperature of the second stage wash water to a temperature that is favorable for the washing in zone e.

It is to be understood that the foregoing description is illustrative rather than limiting on the invention, and that various changes and modifications may be made without departing from the spirit of the invention. Accordingly, the invention is not limited to the specific types of chemicals suggested for use in the two digestion stages, but can equally well be applied to presently lknown chemicals, and may be susceptible to use with chemicals and procedures yet to be developed. Hence, the invention is applicable to substantially any two-stage digesting or bleaching or combined digesting-bleaching method which can be performed in one and the same vessel with regard to the restrictions involved.

It is only necessary according to the invention that the two stage processes performed therewith must be performed at the same pressure, whereas different temperatures may -be chosen, as it is very well possible to maintain a temperature of the first stage that is considerably higher than the temperature of the second stage and vice versa. According to the invention it is also possible to separate the spent digesting liquors of the two stages from each other, in order to recover the chemicals of one or both stages separately. Further, if recovery is not desired or if the two digesting liquors are miscible so that they can be recovered in common after mixing them, the conduits connected to the sieves 33 and. 41 may be connected together by a conduit 69, whereby the recovery of heat is also simplified. Other modifications are also contemplated as coming within the scope of the invention as defined in the claims appended hereto wherein:

I claim:

1. A method for performing two successive treatments of fibrous material in two mutually exclusive treatment stages spaced axially from one another within a single vessel, comprising the steps of:

supply cellulosic lfibrous material and a first chemical treating liquid to the first of said treat-ment stages, remote from the second of said treatment stages and impregnating the cellulosic fibrous material |with said first treating l-iquid; continuously moving the impregnated cellulosic fibrous material through the first treatment stage toward the second treatment stage; supplying a second Ichemical treating liquid to said second treatment stage remote from said first treatment stage and forcing said second treating liquid toward said first treatment stages; withdrawing the spent first treating liquid from the first treatment stage near the second treatment stage; withdrawing the spent second treating liquid from the second treatment stage near the first treatment stage; supplying a wash liquid to said vessel bet-Ween said first and second treatment stages; withdraw-ing said wash liquid from said vessel between the near extents of first and second treatment stages, said lwash liquid being supplied in suliicient quantity to 'wash said cellulosic fibrous material substantially free of the first treating liquid before said cellulosic fibrous material enters said second treatment stages thereby preventing intermingling of the first and second treating liquids.

2. The process of claim 1 wherein both chemical treating liquids are digesting liquids.

3. The process of claim 1 wherein the rst chemical treating liquid is a digesting liquid and the second chemical treating liquid is a bleaching liquid.

4. A continuous digester comprising a longitudinal pressure-resistant vessel having a substantially uniform cross section, means for continuously feeding cellulosic fibrous material into an up-stream end thereof and through said vessel; means for discharging the cellulosic fibrous material from a down-stream end thereof; a rst conduit having orifice means for supplying a lirst treating liquid to the vessel and first sieve girdle means down-stream in respect to the movement of the said cellulosic fibrous material of said first conduit orifice means for withdrawing at least a part of the first treating liquid from the vessel; a second conduit having orifice means spaced down-stream in respect to the movement of the said cellulosic brous material from the said rst sieve girdle means for supplying a second treating liquid to the vessels; and second sieve girdle means for withdrawing the second treating liquid from the vessel, the said second sieve girdle means being located down-stream in respect to the movement of the said cellulosic material from the said second conduit having orifice means, a third sieve girdle means disposed between said first and second sieve girdle means for withdrawing part of the first treating liquid and a fourth sieve girdle means disposed between the said second conduit means having an orifice and the said third girdle means for withdrawing at least a part of the said second treating tiuid, the improvement which comprises: a third conduit having orifice means located within said vessel intermediate said third and fourth sieve girdle means, for supplying wash liquid to said vessel intermediate said third and fourth sieve girdle means, and a fifth sieve girdle means disposed between said third conduit orilice `means and said fourth sieve girdle means, the said third conduit oriiice means being adapted to supply wash liquid in amounts suflicient that a liow of wash liquid countercurrently to the ow of cellulosic material and toward said third sieve girdle means results.

5. The apparatus of claim 4 wherein said third conduit having an orifice means is connected to a circulation loop consisting of said fifth girdle means, an exterior conduit having a circulation pump, and an interior conduit connected to said orice means of said third conduit.

6. The apparatus of claim 4 including sixth Sieve girdle means disposed above said fifth sieve girdle means and a seventh sieve girdle means disposed below the said fifth sieve girdle means, said further sieve girdle means being connected to respective intermediate circulation loops lwhereby the rst and second treating liquids are respectively substantially completely withdrawn.

7. The apparatus of claim 4 including a lowermost sieve girdle for withdrawing a second wash liqfuid which is supplied to said apparatus adjacent said discharge means.

References Cited UNITED STATES PATENTS 942,403 10/1910 Pfiel 162-60 1,683,262 9/1928 Richter 162-60 2,772,138 11/1956 Evans 162-19 3,081,218 3/1963 Ambuehl 162-19 3,200,032 8/1965 Richter 162-60 X 3,298,899 1/ 1967 Laakso. 3,303,088 2/1967 Gessner.

HOWARD R. CAINE, Primary Examiner.

U.S. Cl. X.R. 162-59, 60, 237 

